Airbag-restraint systems for passenger vehicles are known. These systems typically include a firing circuit having a squib and at least two normally open inertia switches, all connected in series across a source of electrical energy. The two inertia switches are known in the art as the safing sensor and the front sensor. The safing sensor is located in the passenger compartment and the front sensor is located near the front of the vehicle. Upon closure of the two inertia switches, as occurs during a vehicle crash, electrical current of sufficient magnitude and duration passes through the squib so as to ignite the squib. The squib, when ignited, ignites a combustible gas generating composition or pierces a container of pressurized gas which results in inflation of the airbag.
If the firing circuit in an airbag restraint system is non-operative, the airbag would not be deployed upon the occurrence of a crash condition. Such non-operative conditions, for example, include (i) an open circuit in the series connection of the squib and the two inertia switches, or (ii) a short circuited squib. Also, a nonoperative condition exists if excessive impedance exists in connections between firing circuit components so that upon closure of the inertia switches, the squib does not draw enough current to fire the squib.
Diagnostic test circuits have been developed to monitor the operativeness of the firing circuit portion of the airbag restraint system. If a non-operative condition is detected in the firing circuit, the diagnostic circuits alert the vehicle operator by lighting a warning lamp. Such diagnostic test circuits are disclosed in U.S. Pat. No. 4,835,513 to McCurdy et al., U.S. Pat. No. 4,825,148 to McCurdy et al., and U.S. Pat. No. 4,990,884 to McCurdy et al., all assigned to the assignee of the present invention, and all of which are hereby fully incorporated herein by reference.
These known diagnostic circuits separately monitor certain parameters of various components in the firing circuit, including the two inertia switches, the squib, and the storage capacitor. The diagnostic circuits monitor for both short and open circuits in the firing circuit and perform tests to determine if resistance or capacitance values of firing circuit components are within predetermined limits. Upon detection of an unacceptable condition in the firing circuit, the circuit alerts the vehicle operator by lighting an indicator lamp located in the passenger compartment. Also, the diagnostic circuits in these patents provide for recording a detected out-of-tolerance condition in an electrically erasable programmable read only memory ("EEPROM") for later analysis by a service technician.
A vehicle may be subject to any one of many different types of crash conditions. For example, the vehicle can hit a barrier such as another vehicle. The vehicle may also crash into a pole. Therefore, it is desirable to have a plurality of crash sensors mounted at various locations of the vehicle so as to be able to sense different types of crash conditions. Such plurality of sensors would be electrically connected in parallel so that the airbag is deployed when any one of several types of crash conditions occurs. Even though certain parallel connections are present in the firing circuit, it is desirable to separately monitor the circuit elements of the firing circuit, as much a possible, for diagnostic testing purposes.